The invention relates to a method of detection of radiation, for example high energy electromagnetic radiation such as x-rays and/or gamma rays, or subatomic particle radiation, and to a method of processing of detected radiation data from a semiconductor device for high energy physics applications, such as a detector for high energy radiation. The invention also relates to a detector device embodying the principles of the method. The invention in particular relates to a semiconductor detector device comprising a large direct band gap semiconductor material, for example a group II-VI semiconductor material such as cadmium telluride (CdTe), cadmium zinc telluride (CZT), cadmium manganese telluride (CMT) or the like, for example formed as a bulk single crystal.
Cadmium telluride and similar semiconductor materials have found application in recent years in a variety of high energy physics applications, such as high energy radiation detection. In particular their ability to resolve high energy electromagnetic radiation such as x-rays or gamma rays spectroscopically has been utilised. This may be especially the case where the material is fabricated as a bulk scale single crystal, which has become a more practical proposition as a result of the development of bulk vapour deposition techniques, for example multi-tube physical vapour phase transport methods, such as that disclosed in EP-B-1019568. For example devices may be fabricated for the detection of radiation at a detector from a suitable high energy source either directly or after interaction with an object under test, involving for example transmission, scattering, backscattering, absorption etc.
The high quantum efficiency of CdTe and CZT and CMT makes these materials ideal for high energy spectroscopy applications. However, in high count rate systems deterioration of the energy spectrum due to pulse pile up can become significant. In the case of systems which rely on an I/I0 data analysis (that is, which process data by taking a ratio of the spectrum when an object is present to that when an object is not present) this can be problematic as this effect is not normalised out of an I/I0 measurement owing to the differing count rates in both I and I0. In addition to this, the short shaping time used in this application can result in ballistic deficit effects due to interactions far from the cathode which further degrade the detector resolution. This effect is also not normalised out by and I/Io measurement due to beam hardening in the I spectrum which results in a larger average depth of interaction. Other depth of interaction effects such as incomplete hole charge collection can also result in degraded spectral resolution. This is particularly significant in large volume high energy detectors where the drift lengths of electrons and holes to the read out electrodes are large.
Photon interactions in the detector (events) in this invention can be subject to a number of effects as a result of which the interaction event may be more or less fully collected. In addition to pulses that are substantially fully collected other pulses may be more poorly collected due to effects including without limitation depth of interaction effects within the detector, pulse pile up of very closely piled up pulses, ballistic deficit effects in the readout electronics etc.
Each presents different problems if an accurate pulse height spectrum is to be collected, particularly for an I/I0 or like data analysis, and particularly at high count rate or for large detector thicknesses.